Abstract Differentiation of CD4 T cells into functionally distinct effector subsets has been intensively studied in vitro and the subset-specific roles of CD4 T cells have been also considered important in vivo. TH1 CD4 T cells, which express T-bet and IFN-?, are thought to be important for both adaptive and innate immune responses to infection by intracellular pathogens and cancers. However, since T-bet is expressed in both CD4 and CD8 T cells, and IFN-? is expressed by other cell populations, such as CD8 T cells and NK cells, the conventional conditional knockout or germline knockout approaches have failed to specifically demonstrate the requirement for TH1 cells in vivo. To address this fundamental question, we have developed a new genetic tool and will define the roles of TH1 cells in vivo using viral infection models.